Open coil electric heating elements are well known in the art and are produced in many forms for various uses. This type of heating element may be disposed in ducts for heating air flowing therethrough; or may be used as a dryer element for clothes dryers; or as a heater for dishwashers; as well as for many other uses.
Because of cost limitations, many metal parts of the heater have been formed of ordinary mild steel which is subject to corrosion in some of the moist environments where the heater is used. Heretofore, to avoid corrosion, some of the metal parts have been formed from galvanized steel strip, but this protection is not only costly but does not entirely avoid corrosion because the strip must be cut, formed and punched, which therefore exposes unprotected edges.
Further, in view of the great amount of attention now being concentrated on consumer safety, it is important that the metal frame is of an electrical insulated nature to avoid dangerous shortcircuits in the event a heating coil should break and a portion thereof touch the frame structure. However, because of the high heat generated by open coil electric heaters, it is not practical to form the frame structure of plastic, rubber, or other insulating materials.
Our invention retains the low-cost feature of forming the heater frame structure of mild steel, without the additional cost of galvanizing, and retains the strength and heat resistance of metal, yet provides corrosion resistance and electrical insulating qualities to such frame structure. In accordance with one aspect of our invention, the metal heater parts are fully assembled with the insulator bushings fixed in position, and this assembly is coated with a silicone resin-based coating composition, and thereafter the coiled resistor is threaded through the bushings and final electrical connection to the terminals is made. In another aspect of our invention, the coating is applied to the assembled metal parts of the frame structure, and then the bushings are fixed in position and the resistor threaded therethrough.